Proceedings of the International Conference on Electrical Engineering and Informatics Institut Teknologi Bandung, Indonesia June 17-19, 2007

PLN (Persero) P3B Jawa Bali Region Jakarta & Banten, Mayjend. Sutoyo 1, Jakarta 13640, Indonesia High Voltage Components and Power System, Delft University of Technology, Mekelweg 4, P.O.Box 5031, 2600 GA Delft, The Netherlands
Gas-insulated switchgear (GIS) installations are metal-enclosed high voltage system with well-known high reliability and compact size. However, for GIS installed in tropical countries, several problems have been experienced. In particular high gas leakage, moisture content, dew point, and decomposition products with levels beyond normal known standards. Due to inherent corrosive properties of the combination of SF6, water vapour, and decomposition products, such conditions will affect parts of GIS installations itself. Since presence of high water vapour in tropical circumstances is unavoidable, the probability of this substance to permeate into the inner parts of GIS installations is high. Thus is it important to investigate the effect of typical tropical circumstances on the performance of gas-insulated substations and how to assess its condition. In order to recognize the failure modes, a failure mode, effect and criticality analysis (FMECA) and series of experiments in the laboratory and onsite measurements are conducted. By using statistical analysis norms that used in assessing the GIS conditions can be determined.

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1. Introduction
GIS installations are fully protected system from ambient conditions by means of their metal enclosure. However, problems of, in particular, gas quality are found in GIS installed at tropical countries. Electricity utilities in South East Asia region (3) have experienced the similar problems. In this contribution, the source of data is failure records of 150 kV GIS installations operated by PLN, Indonesian electricity utility, in Java Bali area. In order to find out whether the tropical conditions affect the performance of GIS, an investigation has been conducted. The investigation is started by examining the failures. As a result of these examinations, different failure paths have been obtained. Moreover, parameters that actually represent the condition of GIS are defined. Actual values of these parameters have been measured, both in the laboratory and onsite. By using these measurements result and statistical analysis, norms can be defined. These norms are used as the references in the GIS condition assessment process.

more than 16 years experienced the most failure. 57.9% of pneumatic failure is categorized as major failure and ended up with replacement. Failure investigation showed that chemical stresses took places in the form of oxidation at valves and compression chamber. Oxidation was caused either by high water vapour concentration or irregularly draining of condensed water. Thus it was caused either by nature or human error. In special case, for example outdoor GIS located in industrial area, corrosive pollutant disrupted valves as well. Gas leakage: Gas leakage phenomena can be found at old installation and installations which were improper installed. The rate of leakage per year varies from 0.423% to 7.868%. Root cause of this phenomenon at old installations has not been determined yet and in depth investigation is under development. In case of improper installation, chemical strength took place in the form of corrosion and create fissure at boundaries between bushing and its base as depicted in Fig. 1.

2. Failure examinations
Based on the failure records, failure examinations are conducted. The aims are to find the deterioration mechanisms, the root cause of failures, parameters that represent the GIS condition, and finally the appropriate diagnostics to assess the performance of GIS.

Fig. 1: Trace of corrosion at bushing base surface Main parts in compartments: Compartment failures showed to be independent to operational age. Based on the investigation to the evidences, the failures were due to technical error, improper construction at junctions, and dielectric strength reduction of SF6. The root cause of the latter modes is most possibly due to SF6 mass loss as has been described in previous paragraph. Despite this evidence, more investigation should be carried out to reveal other possible root cause. Failures at terminations: The causes were due to electrical over-stress as a result of unsuccessful field grading. This mode was either due to improper installation (incompatible

2.1 Failure cases
Experienced failures were related to circuit breaker (CB) operating mechanisms, gas leakage, main parts in compartments in general, and terminations. More details are given in the following. CB operating mechanism: The average operational years of the operating mechanisms ranges from 11 to 16 years. In general operated CB at PLN are coming from pneumatic, hydraulic, and spring type. The population of each type is 19.8%, 53.2%, and 27% respectively. Amongst these types, pneumatic operating mechanisms which are operated for

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Proceedings of the International Conference on Electrical Engineering and Informatics Institut Teknologi Bandung, Indonesia June 17-19, 2007

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cable sealing end at GIS side with cable termination at cable side) or improper construction of the termination.

2.2 FMECA
Through the investigation on failure cases there are ten parts that experienced functional failures: compression fluid valve, compression fluid chamber, and elastic pipe between fluid valve and chamber, seal/O-Ring, insulating gas, mechanical structure driving the moving contact of the circuit breaker, conductor junction in the switching compartment, cable termination, and, boundaries between bushing and its base. Stresses, that experienced by the failed parts before breakdown are chemical, mechanical, electrical, and thermal stresses. Chemical stress, in form of oxidation resulted from the reaction between water vapour and corrosive pollutant or water vapour and enclosure and insulation materials, was the most frequent stress as occurred in 36 events. We also deduced that sometime one or more stresses occurred at the same time may head to breakdown, for example: electrical stress due to local enhancement of electric field, thermal stress due to high energy of over current/voltage, and insufficient amount of insulating gas. The overview of found failure paths in GIS installations in Indonesia is depicted in Fig. 2. In order to find out the criticality of the failing parts, the FMECA was performed. The GIS installation is divided into five subsystems based on its function, they are: primary, secondary, dielectric, driving, and mechanical. Found failed parts are classified into these subsystems. Further analysis will be referred to these subsystems. Risks of each failure
VALVE (PNEUMATICHYDRAULIC) SEAL (COMPRESSION FLUID) ELASTIC PIPE (HYDRAULIC) COMPRESSION CHAMBER (PNEUMATIC) HIGH HUMIDITY CHEMICAL STRESSLEAKAGE (25)

mode are quantified against system (equipment or power system), safety, cost, and frequency of occurrences. As a result there are four parts of that considered to hold high risk, they are: the insulation gas, operating mechanism, any kind of junctions, and seal/O-Ring (Table 1).

2.3 The appropriate parameters of GIS
The failure path and FMECA results shows the appropriate parameters used to assess the GIS condition. In general these parameters can be classified into three groups, they are: general condition, switching device, and environmental influence parameters. First, the general condition parameters represent the quality of the insulating gas and early detection of its deterioration. The quality of the gas is measured by the amount of SF6 and other compounds exist inside the compartment at the working pressure. It is presented by gas pressure, dew point, moisture content, purity, and amount of decomposition products. In order to obtain early deterioration indication an additional detection, i.e. partial discharges (PD) presence should be conducted. PD detection in this surveillance is carried out by using UHF and acoustic techniques. Second, the switching device parameters include: the exact time of opening-closing (determined by time response of the coil, motor pump, and angular velocity to move the contact), the process of opening-closing (described by discrepancy test), the condition of contacts (described by contact resistance value).
INSUFFICIENT PRESSURE PROLONG TIME TO ARC EXTINGUISHMENT

Third, the environmental influence parameters explain how the environment affects the performance of the GIS. The key parameters to distinguish between tropical and subtropical conditions are ambient temperature, humidity, and lightning stroke characteristic. In special cases, where for example GIS next to seashore or in the middle of industrial area, additional parameters should also be monitored. The parameters are salinity and pollutant respectively. The summary of appropriate parameters is listed in Table 2. Table 2. The appropriate parameters
General Gas pressure Dew point Moisture content Purity Decomposition products Partial discharges Switching Angular velocity, time to open-close Discrepancy Contact resistance Compression fluid pressure Motor and trip coil current Environmental Ambient temperature Relative humidity Lightning stroke characteristics Pollutant Salinity (to GIS next to seashore)

distribution to actual probability distribution of measuring data and followed by norms setting. For PD, pattern recognition should be done before calculating the risk level of the PD. PD activities are affected by shape of voltage applied (4), but they have not been proved to be affected by ambient condition. Therefore we decided not to discuss it in this paper.

3.1 Measurements data arrangement
As the FMECA result proves that the insulating gas and the junction (bushing-base) are the two most critical components, monitoring on their conditions is the priority. Insulating gas is part of dielectric subsystem and monitored by measuring the general condition parameters. Junction of bushing-base is part of primary subsystem and its failed condition can be monitored by checking the gas insulation quality as well. In the case of the first critical component, the general condition of insulating gas is distinguished into 3 application categories: CB, bus sections, and other compartments. The classification is based on arc presence risk and each category has its own permissible limit value. As a consequence, we have ten general condition parameters as listed in Table 3. As the first approach, the structure of GIS (one enclosure per phase or per three phases) is not taken into account since it does not affect the quality of the insulating gas. The probability of occurrence of each value for each parameter is calculated and as a result histograms of all ten parameters and the average values of each parameter are found (Table 3).

3. Norms applied in tropical conditions
After finding out the major failure paths and diagnostic techniques to assess the condition of the GIS, norms that will be used as the base assessment parameter should be defined. Before determining norms value, the utility defines the numbers index condition. We use three level of index condition, named 1 for bad condition, 6 for moderate condition, and 9 for good condition. The boundary values from 9 to 6 and 6 to 1 are obtained from onsite and laboratory measurement results analysis. A tool is needed to process the measurement results and translate them into index numbers. The indexing tool development is started by arrange the measuring data (i.e.: measured parameters listed in Table 2) based on its occurrence frequency. Next, we attempt to acquire the fittest

3.2 Setting norms
Determination of norms is done by using the following order: finding the fittest distribution for each gas quality parameter by means of maximum likelihood estimator (MLE) and goodness of fit method, setting theoretically the boundary values, and adjusting the boundary values to the operating circumstances.

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Proceedings of the International Conference on Electrical Engineering and Informatics Institut Teknologi Bandung, Indonesia June 17-19, 2007

compartments. For bus section compartment we obtain the same value as determined by the manufacturers, i.e. 500 ppmv for bus section and 800 ppmv for other compartments. For purity the boundary from 6 to 1 could be changed to 97% as this has determined in IEC 326. No necessary adjustment for decomposition products boundary values is as we obtain the same value with CIGRÉ did. The adjusted norms are listed in Table 4. Table 4: Norms after adjusting to tropical conditions
Parameters gas pressure leakage for CB gas pressure leakage for other compartments dew point for CB dew point for bus sections dew point for other compartments moisture content for CB ppmv moisture content for bus sections ppmv moisture content for other compartments ppmv purity level decomposition products 1 to 6 border 6 to 9 border Unable to be determined 7.875% 1% 0 0 0 1000 1000 1000 97% 2105.3 -10 -10 -5 400 500 800 99.74953% 999.8644

We limit our distribution functions to the most usual ones applied in failure statistic: Normal, Weibull, Lognormal, and Exponential distributions. As the calculation tool we use MATLAB 7.1. As a result of the calculation, we find out that we are not able to determine the distribution function for “gas pressure leakage for CB” due to insufficient amount of data. We also learnt that decomposition products and dew point may reflect the ageing of the insulating gas, as the beta parameter higher than 1.0. Setting the theoretically norms of the boundary values for each gas quality parameter usually based on distribution parameter σ. Hence the border between 1 and 6 is at 2-σ (95%) and the border between 6 and 9 is 1-σ (67%). Unfortunately this border value is not always true in practical world. It should be checked with theoretical permissible limit values and adjusted to tropical conditions. Parameters which affected by ambient condition (i.e.: ambient temperature) are: dew point and moisture content. At one value of SF6 gas working pressure, the higher the ambient temperature is, the higher the dew point is allowed. As ambient temperature is getting higher more moisture is driven from the solid insulating material into SF6 (5), (7). As a result higher moisture content can be observed. The boundary value given by the manufacturer or international standard is applicable if the ambient temperature is 20 ºC. Since the average value of ambient temperature is 30.26 ºC, the boundary value should be corrected. We compare the calculated value with the standards given by IEC 326 (purity level), CIGRÉ 23.10 TASK FORCE 01, May 1997 (decomposition products), manufacturer references, and IEEE as in (5) (dew point and moisture content). As a result, we have to adjust the several parameters. Gas leakage, the boundary from 9 to 6 should be changed to 1% (4), (5) as it is guaranteed by the manufacturers that no leakage higher than 1 % per year. Dew point, the calculation results in a very low boundary values. The limitation is too strict considering the higher the temperature, the higher the permitted dew point. Therefore we could change the 6 to 1 boundary value to the most extreme value, i.e. 6 ºC as stated in (7) or 0 ºC to keep it save. For 9 to 6 boundaries we can use the manufacturers’ references, e.g. -10 ºC for CB or -5 ºC of other compartments and bus section. The boundary values of moisture content for CB are again too strict. We calculate the boundary based on the dew point. Thus the boundary value of 9 to 6 is shifted to 1000 ppmv and 6 to 1 to 400 ppmv for CB

4. Conclusions
Insulating gas and bushing-base junction are the two most important and critical component of a GIS. Stresses due to chemical, electrical, thermal, mechanical can be found in the failure path of GIS. Despite of high value of humidity and dew point, the GIS are still working fine. It is proved by keeping its arc quenching ability during opening session of a CB. Setting the norm for dew point and moisture content is quite tricky since its dependency on ambient temperature. To that purpose we have to measure and find out the ambient temperature average as the guide to determine the maximum permissible value of dew point and subsequently the moisture content. Further adjustment of boundary values should be done based on possible deterioration and failure modes process.